I don't know because return format is wrong or form's load method has bug?
I tried {"MyDate":"2013-2-28"}, {"MyDate":"2013/2/28"}, {"MyDate":"2/28/2013"}, or some other format, but it cannot work.
At last, I do it as below:
$.get('formdata.asp?id='+myId, function(data){
$myDatabox.val(data.dateTime).datebox('setValue', data.dateTime);
$myForm.form('load', data);
},'json')